What is meant by focal length? <!-- Aerial Photographs -->
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USGS Frequently Asked Questions
Question:
What is meant by focal length?
Answer:
Focal length is the distance measured from camera lens to film.
Knowledge of the focal length used, along with the altitude of the
photographic aircraft, makes it possible to determine the scale of
the aerial photograph (assuming land uniformity). For example, with
a camera focal length of six inches, a flight altitude of 6,500 feet
above mean sea level, and an average ground elevation of 1,500 feet,
the representative fraction would be computed as follows:
0.5 ft. divided by (6,500 ft. - 1,500 ft.) = 1:10,000 scale
